PROJECT SUMMARY Specific language impairment (LI) is a common child learning disorder that can persist into adulthood and that affects approximately 7% of children in the United States. LI is characterized by marked deficits in acquiring aspects of language including the sound structure of language, grammatical morphology, and the syntactic rules that govern word order. Individuals who are diagnosed with LI in childhood often show increased risk of developing other disabilities including learning disability and reading disability, in addition to being at increased risk of behavioral problems and failure to thrive in academic environments. The long-term goal of the proposed research is to optimize a theoretical understanding of the neural and behavioral etiology of LI. Although a substantial amount of high quality behavioral and brain-based research has been conducted and informed LI treatment, this research has often made assumptions about the intact nature of lower-level phonetic skills in LI. The first step towards the long-term goal is to test the hypothesis that LI reflects deficits in the neural representation and functional plasticity of phonetic category structure. Aim 1 uses fMRI neuroimaging to examine how the brain represents phonetic category structure. We hypothesize that (1) children and adults with LI will show evidence of rigidity in the temporoparietal brain regions that process phonetic category structure and (2) phonetic category structure becomes more defined across development such the neural coding of category goodness is more graded in adults compared to children. Aim 2 uses behavioral tasks to examine functional plasticity of phonetic category structure with respect to signal-driven and lexically-driven learning mechanisms. We hypothesize that children and adults with LI will show a decreased ability to dynamically modify the mapping to phonetic categories as a consequence of exposure to different distributions of phonetic variability. We also hypothesize that language ability mediates how lexical information is used to dynamically adjust perceptual representations and predict that children and adults with LI will show an attenuated ability to modify the mapping to speech sound categories using lexical information. With respect to previous research that has focused on characterizing LI in a static perceptual space, the innovation in this proposal is examining the neural and behavioral basis of the dynamic processes that support plasticity of phonetic category structure. The proposed research addresses important theoretical and clinical problems in the fields of language science, neurobiology, language impairment, and clinical practice including the need for a theoretical understanding of how phonetic category structure is formed and maintained across the developmental trajectory. Achieving these aims will lay important groundwork for improved specification of etiology of LI, which can be used to develop more targeted rehabilitation protocols, as well as for future neuroimaging studies that will be able to further explicate the neural basis of LI and functional plasticity.